This invention relates to a method for use in controlling the operation of a steerable drilling system. The method is particularly suitable for use with a rotary steerable system, but may be used in other types of steerable drilling system used in the formation of subterranean wells. In particular, the invention relates to a method of predicting how a drilling system will operate, respond or react to various operating conditions and changes therein.
One type of rotary steerable system comprises a downhole assembly including a drill bit. The drill bit is carried by a drill string which is rotated typically by a well head located drive arrangement. A bias unit is included in the downhole assembly, the bias unit including a plurality of hinged pads moveable between extended and retracted positions. The pads are moved hydraulically using drilling fluid under the control of a valve arrangement. The valve arrangement is designed to permit control over the pads such that, when desired, the pads can be moved to their extended positions in turn as the bias unit rotates. By appropriate control over the pads, the bias unit can be operated to apply a sideways load on the drill bit which in turn will cause the formation of a curve in the well bore being drilled. The orientation of the curve will depend upon how the bias unit is controlled.
It has been found that a number of factors must be taken into account when controlling the operation of a rotary steerable system. For example, the rate of change of direction of the bore hole being formed in response to the application of a given command signal to the bias unit depends upon several factors associated with the drilling system, for example rotary speed, weight on bit, rate of penetration and several factors associated with the formation being drilled, for example the dip and azimuth of bedding planes. As a consequence, it is common for well bores drilled using steerable drilling systems to deviate from their desired paths. Such well bores may be of tortuous form containing many dog legs. Depending upon the orientation of the curves formed in the well bore, water or gas may tend to collect in the curves. Such accumulation of water or gas may impair subsequent use of the well bore in the extraction of oil.